The proposed research will define the essential structural and functional elements of thrombopoietin necessary for stability, activity and efficient secretion of the protein. Thrombopoietin (TPO) is a recently discovered cytokine critical for early development of bone marrow cells and late development of platelets, the megakaryocyte fragments which initiate blood clotting. As a therapeutic agent Thrombopoietin is likely to be useful for the support of patients with bone marrow failure, both naturally occurring and iatrogenic in origin. Fundamental studies of structure and function of TPO will enhance understanding of this protein in normal physiology and disease states and may lead to identification of TPO antagonists (potential new therapeutic agents for primary and acquired hematologic diseases). TPO is composed of two regions, one similar to several other cytokines, and a second which appears to be unique amongst hematopoietic hormones. The proposed research will identify critical functional and structural residues of the first region (TPO1-152), the receptor binding domain. The remaining TPO residues (153-332) appear to function as a prosequence, first enhancing secretion of TPO and ultimately inhibiting potency of TPO-152. The TPO glycan tail of (TGT) enhances TPO secretion even when expressed as a separate polypeptide, making possible the design of experiments to characterize it's activity. The studies proposed will examine the subcellular locus of TGT function. the portions of TGT necessary for bioactivity, and the biochemistry of interaction of the two regions. Characterization of both of these TPO regions will enhance our understanding of the production, activity and regulation of TPO. The University of Washington provides a strong research environment for this proposal with many faculty members engaged in related studies (hematopoiesis, biochemistry, molecular biotechnology) whose advice and guidance will be available, as well as ongoing lectures and courses in relevant areas. Dr. Kaushansky and the advisory panel, formed specifically for the aims of this proposal will oversee the applicant's research and academic progress. A defined plan of training is presented during which the applicant will develop the experience and skills necessary for success as an independent investigator in Hematology